1. Field of the Invention
The present invention relates to an electronic endoscope.
2. Description of the Related Art
In recent years, the following technology is being put into practical use. That is, an electronic endoscope having a solid-state imaging device has been used to diagnose lesions of living tissues based on images obtained by applying excitation light onto a site, under observation, of a living tissue and imaging autofluorescence produced in the living tissue by the excitation light or fluorescence of an agent injected into a living body.
With respect to the autofluorescence of living tissue, when excitation light at a wavelength of about 405 nm is applied onto the living tissue, for example, normal tissue will produce green fluorescence at a wavelength of about 520 nm. In contrast, a lesion tissue such as cancer will not produce fluorescence or the fluorescence produced will be weak. Therefore, the lesion can be diagnosed.
With respect to the fluorescence of the agents, fluorescent materials such as hematoporphyrin derivatives and ALA (δ-aminolevulinic acid) are used. These fluorescent materials will produce red fluorescence at a wavelength of about 630 nm when excitation light at a wavelength of about 405 nm is applied thereto. These fluorescent materials tend to accumulate in lesion tissues such as cancer. Therefore, the lesion can be diagnosed.
Various types of electronic endoscopes have been proposed to perform the above-mentioned fluorescence observation and normal observation to apply white light onto a living tissue and image reflected/scattered light (for example, see JP Hei.8-140928 A (corresponding to U.S. Pat. Nos. 6,099,466 and 6,471,636), JP Hei.9-70384 A (corresponding to U.S. Pat. Nos. 6,099,466 and 6,471,636) and JP 2003-79570 A (corresponding to US 2003/0050532 A).
JP Hei.8-140928 A describes an electronic endoscope provided with two solid-state imaging devices at a distal end of the endoscope for normal observation and fluorescence observation.
JP Hei.9-70384 A describes an electronic endoscope provided with two solid-state imaging device at a distal end of the endoscope for normal observation and fluorescence observation. A supersensitive solid-state imaging device is used as the solid-state imaging device for fluorescence observation.
JP 2003-79570 A describes an electronic endoscope using one solid-state imaging device with a variable charge multiplication factor. The one imaging device performs normal observation and fluorescence observation by changing the charge multiplication factor between the normal observation and the fluorescence observation.
When two solid-state imaging device for the normal observation and the fluorescence observation are provided at the distal end of the endoscope as described in JP Hei.8-140928 A and JP Hei.9-70384 A, it is difficult to reduce a size of the distal end of the endoscope.
Furthermore, an intensity of the autofluorescence of the living tissue or the fluorescence of the agent is extremely weak compared to that of the excitation light reflected/scattered by the living tissue. Therefore, the endoscopes described in JP Hei.8-140928 A and JP Hei.9-70384 A have a filter for passing only the fluorescence which is disposed between the solid-state imaging device for fluorescence observation and an objective optical system for guiding feedback light (reflected/scattered light or fluorescence) from the living tissue to the solid-state imaging device. The endoscope described in JP 2003-79570 A has a filter for cutting the excitation light which is disposed between the solid-state imaging device and an objective optical system.
In the endoscope described in JP 2003-79570 A which performs normal observation and fluorescence observation with the same solid-state imaging device, since the excitation light cutting filter is disposed between the solid-state imaging device and the objective optical system, color information corresponding to the wavelength of the excitation light is lost in normal observation.
Furthermore, the intensity of the autofluorescence of the living tissue or the fluorescence of the agent is extremely weak even compared to that of the reflected/scattered light imaged in normal observation. Then, in order to obtain images having appropriate brightness for both the normal observation and the fluorescence observation, the endoscope described in JP Hei.9-70384 A uses a supersensitive solid-state imaging device for the fluorescence observation, while that described in JP 2003-79570 A uses a solid-state imaging device with the variable charge multiplication factor. However, both of them are expensive.